This process will allow you to enable JIT (Just In Time compiling) on Eclair (2.1) builds.
This is based on Jus10o's guide on XDA.

Tested on Motorola CLIQ running Eclair2CLIQ Beta 1 with my root fix.

Step 1: Download
Download the files needed here.

Step 2: Extract Files
Make a folder called ~/tmp (or something else if you prefer). Extract the contents of the archive you downloaded to that folder.

Step 3: Push Files
Open up terminal and type
cd ~/tmp
adb push dalvikvm /sdcard/jit/
adb push libdvm.so /sdcard/jit/
adb push libnativehelper.so /sdcard/jit/

Step 4: Modify build.prop
Pull your current build.prop
cd ~/tmp
adb pull /system/build.prop .
Open in a text editor and input the line "dalvik.vm.execution-mode=int:jit" at the very end of the file.

Push build.prop back
adb push build.prop /sdcard/jit/

Step 5: Backup
adb shell
cp /system/bin/dalvikvm /sdcard/dalvikbackup/
cp /system/lib/libdvm.so /sdcard/dalvikbackup/
cp /system/lib/libnativehelper.so /sdcard/dalvikbackup/
cp /system/build.prop /sdcard/dalvikbackup/

Step 5: Copy the new files to their locations
adb shell
cd /sdcard/jit/
cp -f dalvikvm /system/bin/
cp -f libdvm.so /system/lib/
cp -f libnativehelper.so /system/lib/
cp -f build.prop /system/

Step 6: Change Permissions and reboot into recovery
adb shell
chmod 755 /system/bin/dalvikvm
chmod 644 /system/lib/libdvm.so
chmod 644 /system/lib/libnativehelper.so
chmod 666 /system/build.prop
sync
reboot

Follow the standard procedure to get into recovery and wipe only dalvik-cache.
Reboot.

Step 7: Enjoy JIT!

To go back to non-JIT:

adb shell
cd /sdcard/dalvikbackup
cp -f dalvikvm /system/bin/
cp -f libdvm.so /system/lib/
cp -f libnativehelper.so /system/lib/
cp -f build.prop /system

chmod 755 /system/bin/dalvikvm
chmod 644 /system/lib/libdvm.so
chmod 644 /system/lib/libnativehelper.so
chmod 666 /system/build.prop
sync
reboot

Enter recovery; wipe dalvik-cache; reboot. You are now JIT-less.